In a distributed packet exchange network such as the Internet, the packet distribution format is classified into several types according to the destination designation method. Following packet distributions are employed in IPv4 used in the present Internet and IPv6 which is standard of next-generation Internet.                1) Unicast distribution in which the destination is the address expressing the single interface,        2) Multicast distribution (hereafter referred to as multicast) in which the destination represents a group of a plurality of interfaces, and copy of packet (in the explanation below ‘packet’ may mean ‘packets’) is distributed to all of them, and        3) Anycast distribution in which the destination represents a group of the plurality of interfaces, and copy of packet is distributed to any one of them.        
The present invention relates to the above-mentioned multicast. In multicast it is possible to distribute same data efficiently to a plurality of nodes. Therefore, this type of distribution is utilized in the fields of multimedia data broadcast, multi-point audio and video conference, etc.
An example of realizing multicast in IPv4 will be explained here as a prior art. In IPv6 of which standardization is being presently proposed, the method of realization is nearly the same as explained here. Actual execution of multicast requires three steps described in detail below. That is:                1) Address assignment of multicast,        2) Request of route setting, and        3) Packet distribution.1) Address Assignment of Multicast:—        
In IPv4, of the IP (Internet protocol) address space of four octets, one-sixteenth is assigned for the space of multicast called class D. The address of class D is determined so that the higher four bits may start with 1110. A sender of multicast of packet is assigned with a multicast address by the IANA (Internet Assigned Number Authority) and ICANN (Internet Corporation for Assigned Names and Numbers), one each for every group of address node for multicast.
2) Request of Route Setting
The sender, while communicating with each one of the receivers, must request preliminarily route setting to be used in the actual packet distribution (described below) to all routers on the route to which the multicast packet is distributed.
3) Packet Distribution
IPv6 packet has the header format as shown in FIG. 20 in both unicast and multicast. ‘Version’ in this figure refers to the edition number, ‘Class’ indicates whether the data is video, audio or normal data, and ‘Flow Label’ determines the flow number to be given to the flow.
‘Payload Length’ denotes the data length, and the ‘NextHeader’ tells which protocol in advance. ‘Hop Limit’ indicates the upper limit of the number of repeats to avoid endless hopping of packet in the network.
‘Source Address’ is the sender's address and ‘Destination Address’ is the end address. Finally, the host protocol data is provided.
The transmission node stores and transmits the multicast address assigned to the destination address among them. An intermediate router searches the route table prepared for relaying the packet in a correct direction. The route table is composed as shown in FIG. 21. ‘Network’ row in this figure lists up the networks that can be reached from this route on the Internet.
The network is expressed by the network prefix and mask (net mask). For example, if the prefix is 3FFE:501:1000::, and the mask is 40 (FFFF:FFFF:FF00::), it expresses a network in a range of 3FFE:501:1000:0:0:0:0: to 3FFE:501:10FF:FFFF:FFFF:FFFF:FFFF:FFFF.
‘Destination’ row in FIG. 21 expresses the address neighbor of the route to which the router itself request distribution next for distributing the packet to this network, and the interface for sending out the packet for this purpose. The route table search is to calculate the AND of the address to be searched and net mask in the first place, and find out an item of which result is equal to the network prefix, and the packet is distributed from the interface to the destination neighbor of the searched item (various high speed techniques are proposed for this route table search, and many patents are pending).
When relaying the unicast address, only one destination is always set, and the packet is transferred from the designated interface to the next designated router. Since one next router is always set in all routers from the sender to the receivers, as a result, the packets are distributed on one route.
On the other hand, in the multicast, only one destination is present, or two or more, depending on the router. In the case of one destination, the operation is same as in unicast, but in the case of two or more, the packet is copied and distributed to each one. Accordingly, only one packet at the time of transmission is branched off on the network, and is distributed to a plurality of receivers.
The existing multicast system involves the following problems.
1) Address Assignment
In the case of multicast, one multicast address must be assigned to every group to be distributed to. In the case of the broadcasting type multicast as substitute for the existing television and radio, a permanent address can be assigned, but in the case of communication with dynamically increasing channels such as multi-point television conference relay, it is required to issue multicast address dynamically on every occasion. It requires a uniform rule by the Internet, and in order to match commonly, a certain complicated structure is needed.
2) Router Setting
After multicast address assignment, items of multicast addresses must be set in the route table of all routers existing in the route from the sender to the individual destination clients. In the key router of the Internet, this route table is an enormous number. Further, in the destination node group corresponding to the multicast address, members are changed frequently. On every occasion, the route must be calculated again, and the route table must be updated. This processing is also an enormous amount.